Energy flows through an ecosystem in only one direction. Energy is passed from organisms at one
trophic level
or energy level to organisms in the next trophic level. Which organisms do you think are at the first trophic level (
Figure
below
)?

Producers are always the first trophic level, herbivores the second, the carnivores that eat herbivores the third, and so on.

Most of the energy at a trophic level – about 90% – is used at that trophic level. Organisms need it for locomotion, heating themselves, and reproduction. So animals at the second trophic level have only about 10% as much energy available to them as do organisms at the first trophic level. Animals at the third level have only 10% as much available to them as those at the second level.

Food Chains

The set of organisms that pass energy from one trophic level to the next is described as the
food chain
(
Figure
below
). In this simple depiction, all organisms eat at only one trophic level (
Figure
below
).

A simple food chain in a lake. The producers, algae, are not shown. For the predatory bird at the top, how much of the original energy is left?

What are the consequences of the loss of energy at each trophic level? Each trophic level can support fewer organisms.

How many osprey are there relative to the number of shrimp?

What does this mean for the range of the osprey (or lion, or other top predator)? A top predator must have a very large range in which to hunt so that it can get enough energy to live.

Why do most food chains have only four or five trophic levels? There is not enough energy to support organisms in a sixth trophic level. Food chains of ocean animals are longer than those of land-based animals because ocean conditions are more stable.

Why do organisms at higher trophic levels tend to be larger than those at lower levels? The reason for this is simple: a large fish must be able to eat a small fish, but the small fish does not have to be able to eat the large fish (
Figure
below
).

In this image the predators (wolves) are smaller than the prey (bison), which goes against the rule placed above. How does this relationship work? Many wolves are acting together to take down the bison.

Food Webs

What is a more accurate way to depict the passage of energy in an ecosystem? A
food web
(
Figure
below
) recognizes that many organisms eat at multiple trophic levels.

A food web includes the relationships between producers, consumers, and decomposers.

Even food webs are interconnected. All organisms depend on two global food webs. The base of one is phytoplankton and the other is land plants. How are these two webs interconnected? Birds or bears that live on land may eat fish, which connects the two food webs.

Humans are an important part of both of these food webs; we are at the top of a food web, since nothing eats us. That means that we are top predators.

Summary

A food chain describes the passage of energy between trophic levels.

A food web is a set of interconnected and overlapping food chains.

Food webs are interconnected, such as nearby land and a marine food webs.